The thirst for meaning
The penguins themselves symbolize something essential about Mr. Popper's life. Because the penguins bring intrigue and adventure to his life by posing problems for him to solve, he willingly sacrifices his money, his time, his home, his comfort, his reputation, his safety—all for the hope of living an interesting life. This is underscored by the name of the town: Stillwater. He wants to escape the boredom of his small town life.
Life and survival
When the penguins arrive, they are in bad condition. He manages to get a female friend for his male penguin, and they struggle to heal from their mistreatment, eventually procreating. They have twelve penguins that they aid in their survival, and all this points to the meaning of life, because Mr. Popper's life is comfortable. Survival isn't a daily concern of his, but the penguins make him constantly aware of life's meaning, which at least for the penguins, is just survival and family.
Hope and the promise of adventure
This novel is not about an adventure, although Mr. Popper goes through an emotional adventure taking care of penguins. Then, his reward for all his potentially misguided, pointless sacrificing is that he learns that he will one day get to adventure. He gets to go to the North Pole with his adventuring hero, Admiral Drake. It's a dream come true, and it's that promise of surprise adventure that gives him hope.
